The present invention relates to a method for investigating the behavior of particles or droplets in a gas-vapor mixture inside a flow device, which can be used to study cloud droplet dynamic and microphysical processes.
Atmospheric clouds are complicated systems. Many important processes in clouds are still not completely understood. For example, the activation of aerosol particles into drops, the effects of water vapor diffusion on the growth or shrinking of aerosol particles/drops, or the adsorption (or absorption) and desorption of trace gasses in moist aerosol particles (and the role trace gasses play in the growth or shrinking of aerosol particles/drops) are not completely understood.
In order to obtain a better understanding of the chemical and microphysical processes in clouds, many attempts have been made over the last century and in recent years to study clouds by means of field and laboratory experiments as well as numerical models. Some prior attempts and their corresponding devices can be found in Cziczo, D. J. and J. P. D. Abbatt, xe2x80x9cDeliquescence, efflorescence, and supercooling of ammonium sulfate aerosols at low temperature: Implications for cirrus cloud formation and aerosol phase in the atmosphere,xe2x80x9d J. Geophys. Res., 104(11):13781-13790 (1999); Chuang, P. Y.-S., 1999, xe2x80x9cExperimental and theoretical studies of cloud condensation nuclei,xe2x80x9d Ph.D.-thesis, California Institute of Technology, Pasadena, Calif., p. 157; and Nenes et al., J. Geophys. Res., 106:3449-3474 (2001).
Clouds exist in their natural environmentxe2x80x94the atmospherexe2x80x94which itself is a fairly complicated system. Therefore, it is not a trivial task to study or even produce realistic clouds in a laboratory. For example, all of the prior devices referenced above depend on measurements of one or a few water supersaturations, and then require theoretical modeling to predict, based on the measurements, what might happen in real clouds. Thus, in these devices, it is not possible to accurately simulate the time dependence of drop growth or shrinking as it occurs in the lower atmosphere (troposphere). Further, the prior devices are based on the assumption that above a certain supersaturation level aerosol particles are spontaneously activated into droplets. Thus, these prior devices do not consider, and thus not simulate, subsaturation conditions in which aerosol particles remain unactivated.
Still further, existing methods as used in the prior devices for studying the growth or shrinking of particles/droplets in a vapor field inside a flow tube or static chamber also have the disadvantage that their thermodynamic system parameters cannot be defined with sufficient accuracy. For example, these methods are substantially dependent on the accuracy of their temperature and vapor pressure control devices, which are unfortunately not capable of setting these parameters as accurately as one would desire. Usually, the vapor concentration and saturation field inside a flow tube or static chamber are determined with analytical or numerical models, which depend on the experimental data, such as temperature differences, as boundary conditions. Thus, uncertainties in the thermodynamic system parameter control devices, such as the temperature control device, negatively influence the model predictions. Typically, either thermoelectric elements or thermocouples are employed to control the temperature of either the wall of a flow tube or the temperature of a cooling liquid in a double jacket wrapped around a flow tube or static chamber. State of the art temperature control units manage to achieve an accuracy in the range of +/xe2x88x920.1xc2x0 K to 0.05xc2x0 K. This still leads to uncertainties in the vapor field inside the flow tube or static chamber, with the consequence that the thermodynamic conditions under which particles/droplets are to grow cannot be adequately defined. As a result, there are large uncertainties as to the resulting sizes of the particles/droplets. For example, for water vapor, such temperature deviations (uncertainties) may even lead to a completely uncontrolled system, which may reach either supersaturated or subsaturated condition in the limits of the accuracy of the temperature control.
Also, in these prior devices, it is not possible to detect the influence of trace gasses upon the growth or shrinking of particles/droplets in a gas-vapor mixture.
A need exists for a method and apparatus that accurately simulate the time dependence of both subsaturation and supersaturation conditions in an atmospheric cloud, in order to permit the study of both unactivated and activated particles or droplets. Such method and apparatus would be suited for simulating the time dependence of drop growth or shrinking as it occurs in the lower atmosphere (troposphere). Preferably, such method and apparatus should allow for accurate control of thermodynamic system parameters so as to establish a well-defined vapor concentration and saturation field inside a flow device. Further preferably, such method and apparatus should permit observing the influence of trace gaseous species on droplet or particle growth or shrinking.
The present invention provides a method for investigating the behavior of particles or droplets (e.g., NaCl, soot, ammonium sulfate, sulfuric acid, biological particles such as pollen) in a gas-vapor mixture inside a flow device, which can meet all the needs described above.
The method generally includes five steps. First, a flow device is provided, including an internal standard with known properties and behavior therein. Specifically, particles or droplets with known and/or defined size, chemical composition, concentration (e.g., number concentration), and growth or shrinking behavior in a gas-vapor mixture are used as the internal standard, and included in or injected into the flow device.
Second, various thermodynamic system parameters of the flow device are adjusted based on the observed behavior of the internal standard. For example, the particle or droplet size profile (i.e., how various particle sizes are spatially distributed) and the concentration profile (i.e., how the concentration is spatially distributed) of the internal standard are determined by measurement using any suitable space and/or time resolved measurement methods, preferably optical methods, as well known in the art. Based on the measured behavior of the internal standard, the thermodynamic system parameters of the flow device are adjusted and/or controlled in such a way that a desired particle or droplet size profile of the internal standard is achieved. In other words, the parameters are adjusted so that the particles or droplets of the internal standard will grow as they should in accordance with their known activation and growth behavior. Thus, this adjustment of the thermodynamic system parameters, in turn, achieves a well-defined vapor concentration and saturation field inside the flow device.
Third, particles or droplets to be investigated are injected into this well defined flow device, either independently or with the internal standard.
Fourth, the actual particle or droplet size of the injected particles or droplets is measured. As before, this is accomplished by using any suitable space and/or time resolved measurement methods, preferably optical methods.
Fifth, the behavior, for example the activation and growth or shrinking behavior of the particles or droplets to be investigated, is determined based on the measured size of the particles or droplets and the adjusted system parameters, using a mathematical/numerical model.
Various applications or modifications of a method according to the present invention are possible. For example, in the third step, the particles or droplets to be investigated may be injected into the flow device together with trace gasses. In this case, the particles or droplets to be investigated are with known and/or defined size, chemical composition, concentration (e.g., number concentration), and growth or shrinking behavior. Thereafter, in the fourth step, as before, the particle or droplet size of the injected particles or droplets to be investigated is measured. In the fifth step, the behavior of the particles or droplets to be investigated is determined based on their measured size and the adjusted system parameters using a mathematical model. Specifically, in this case, any effects of the trace gasses on the activation and growth or shrinking behavior of the particles or droplets can be determined, by comparing the determined behavior against the predefined growth or shrinking behavior of the particles or droplets without the presence of trace gasses.
As another application example, in the third step, the particles or droplets to be investigated may be with known or predefined concentration (variable). In the fourth step, as before, the particle or droplet size of the injected particles or droplets is measured. Finally, in the fifth step, the behavior of the particles or droplets to be investigated is determined based on their measured size and the adjusted system parameters using a mathematical model. Specifically, in this case, any effects of the concentration (e.g., number concentration) of the particles or droplets on the activation and growth or shrinking behavior of the particles or droplets can be determined by repeating the method with each time varying the concentration of the particles or droplets.
According to one aspect of the present invention, the flow device suited for carrying out a method of the present invention is a flow tube, preferably a laminar flow tube.
A method of the present invention is particularly suited for simulating both subsaturation and supersaturation conditions in an atmospheric cloud, including both unactivated and activated particles or droplets, to permit a realistic study of cloud droplet dynamic and microphysical processes. According to the present invention, the thermodynamic system parameters of the flow device can be accurately controlled and defined so as to establish a well-defined vapor concentration and saturation field, thereby closely simulating the thermodynamic conditions of real atmospheric clouds. Furthermore, the invention permits injecting trace gasses into the flow device together with the particles or droplets to be investigated, thus allowing for the investigation of the role of, for example, organic compounds in the formation of clouds.